1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (hereunder sometimes called simply a “photoreceptor”), to a manufacturing method therefor and to an electrophotographic device, and relates specifically to an electrophotographic photoreceptor that is formed principally of a conductive substrate and a photosensitive layer containing an organic material, and is used in electrophotographic printers, copiers, fax machines and the like, and to a manufacturing method therefor and an electrophotographic device.
2. Background of the Related Art
The basic structure of an electrophotographic photoreceptor comprises a photosensitive layer with a photoconductive function on a conductive substrate. In recent years, organic electrophotographic photoreceptors using organic compounds as functional components for producing and transporting charge have been subjects of active research and development because of their diversity of materials, high productivity and safety among other advantages, and they are being applied to copiers, printers and the like.
In general, a photoreceptor must have the function of holding a surface charge in a dark place, the function of receiving light and generating charge, and also the function of transporting the generated charge. Such photoreceptors include monolayer photoreceptors provided with a monolayer photosensitive layer having all these functions, and stacked (functionally separated) photoreceptors provided with a photosensitive layer comprising a stack of functionally discrete layers: primarily, a charge generating layer that serves the function of generating charge during photoreception and a charge transport layer that serves the functions of holding a surface charge in a dark place and transporting the charge generated in the charge generating layer during photoreception.
The photosensitive layer is normally formed by dissolving or dispersing a charge generating material, a charge transport material and a resin binder in an organic solvent to obtain a coating liquid that is then applied to a conductive substrate. In these organic electrophotographic photoreceptors, polycarbonates that are highly flexible, transparent to light exposure and resistant to friction with the paper and the blade used for toner removal are often used as resin binders in the layer forming the outermost surface in particular. Of these, bisphenol Z polycarbonate is widely used as a resin binder. Techniques using this polycarbonate as a resin binder are described for example in Japanese Patent Application Laid-open No. S61-62040 and the like.
Currently, most electrophotographic devices are so-called digital devices using a monochromatic exposing source such as an argon, helium-neon or semiconductor laser or a light-emitting diode, whereby images, words and other information are digitalized and converted to an optical signal, and exposed on a electrically charged photoreceptor to thereby form an electrostatic latent image that is then developed with toner.
Methods of charging the photoreceptor include non-contact charging systems using scorotrons and other charge devices that do not contact the photoreceptor, and contact charging systems using charge devices with semiconductive rubber rollers and brushes that do contact the photoreceptor. The advantage of a contact charging system over a non-contact charging system is that little ozone is generated because the corona discharge occurs very near the photoreceptor, so that little applied voltage is required. Thus, this system is favored in medium-sized and small devices in particular because it provides an electrophotographic device that is compact, inexpensive and environmentally friendly.
The most common methods for cleaning the photoreceptor surface include scraping with a blade and simultaneous developing/cleaning processes. In the case of blade cleaning, untransferred residual toner on the surface of the organic photoreceptor is scraped off with a blade, and the toner can then be collected in a waste toner box or returned to the developing machine. The difficulty with cleaning by this blade scraping system is that space is required for the toner collection box and recycling, and it is necessary to monitor the amount of toner in the toner collection box. If paper dust and external additives accumulate on the blade, moreover, they can damage the surface of the organic photoreceptor, shortening the life of the electrophotographic photoreceptor. Thus, the toner is sometimes collected in the developing process, or a means for magnetically or electrically suctioning residual toner adhering to the surface of the electrophotographic photoreceptor is installed immediately before the developing roller.
When using a cleaning blade, moreover, the rubber hardness and contact pressure must be increased in order to improve the cleaning properties. This promotes wear of the photoreceptor, causing fluctuations in potential and sensitivity, and leading to image abnormalities and problems of color balance and reproducibility in the case of color devices.
In the case of a cleaningless system in which development and cleaning are performed together by a developing device using a contact charging mechanism, toner with a fluctuating charge quantity is produced in the contact charging mechanism. Another problem is that when the toner is contaminated by a small quantity of reverse-polarity toner, these toners cannot be sufficiently removed from the photoreceptor, and contaminate the charging device.
The surface of the photoreceptor may also be contaminated by ozone, nitrogen oxides and the like produced during charging of the photoreceptor. In addition to image deletion caused by the contaminants themselves, adhering substances may reduce the lubricity of the surface, making it easier for paper dust and toner to adhere to the surface and cause blade noise, burr, surface scratches and the like among other problems.
In order to increase the transfer efficiency of the toner during the transfer step, moreover, attempts have been made to improve transfer efficiency and reduce residual toner by optimizing the transfer current for the properties of the paper and the temperature and humidity environment. As a result, organic photoreceptors with improved toner release properties and organic photoreceptors with reduced transfer effect are needed as organic photoreceptors suited to such processes and contact charging systems.
To resolve these problems, various methods have been proposed for improving the outermost layers of photoreceptors. For example, Japanese Patent Application Laid-open No. H1-205171 and Japanese Patent Application Laid-open No. H7-333881 propose methods for adding fillers to the photoreceptor surface layer in order to improve the durability of the photoreceptor surface. However, it is difficult to disperse the fillers uniformly with these methods of dispersing the filler in the film. Filler aggregates also occur, film transparency is reduced, and the filler scatters the exposure light, causing irregularities of charge transport and charge generation and detracting from the image characteristics. One method of improving filler dispersibility is to add a dispersant, but in this case the dispersant affects the photoreceptor characteristics, which are difficult to reconcile with filler dispersibility.
In the method disclosed in Japanese Patent Application Laid-open No. H4-368953, polytetrafluoroethylene (PTFE) powder or other fluorine resin powder is included in the photosensitive layer. In the method disclosed in Japanese Patent Application Laid-open No. 2002-162759, an alkyl denatured polysiloxane or other silicone resin is added to the outermost layer of the photoreceptor. However, in the method of Japanese Patent Application Laid-open No. H4-368953 the PTFE powder or other fluorine resin powder has poor solubility in the solvent or poor compatibility with other resins, causing phase separation and light scattering at the resin boundary. Therefore, the sensitivity characteristics have not been adequate for a photoreceptor. In the method of Japanese Patent Application Laid-open No. 2002-162759, the problem has been that continuous effects are not obtained because the silicone resin bleeds on the surface of the coating film.
To solve these problems, Japanese Patent Application Laid-open No. 2002-128883 proposes a method for improving wear resistance whereby a resin having a polysiloxane structure added to the terminal structures is used in the photosensitive layer. Japanese Patent Application Laid-open No. 2007-199659 proposes a photoreceptor containing a polycarbonate or polyallylate made of a phenol raw material containing a specific siloxane structure. Japanese Patent Application Laid-open No. 2002-333730 proposes a photoreceptor containing a polysiloxane compound comprising carboxyl groups in a resin structure. Japanese Patent Application Laid-open No. H5-113670 proposes a photoreceptor in which the photosensitive layer uses a polycarbonate the surface energy of which has been reduced by the inclusion of a silicone structure. Japanese Patent Application Laid-open No. H8-234468 proposes a photoreceptor containing a polyester resin comprising polysiloxane structural units. Further, Japanese Patent Application Laid-open No. 2009-098675 proposes a photoreceptor using an electrophotographic photoreceptor resin composition containing a polycarbonate resin and a polysiloxane group-containing A-B block copolymer with a specific structure as a resin binder, but when added as a polysiloxane group-containing copolymer, this copolymer tends to segregate in the surface layer of the photoreceptor, and it has been difficult to ensure a lasting low-friction coefficient.
Methods have also been proposed for forming surface protective layers on the photosensitive layer with the aim of protecting the photosensitive layer and improving mechanical strength and surface lubricity. The problems with these methods of forming surface protective layers have been the difficulty of forming a film on a charge transport layer, and the difficulty of achieving both charge transport characteristics and charge retention functions.
Thus, various techniques have already been proposed for improving photoreceptors. However, the techniques described in these patent documents have not been adequate for maintaining continuously low friction resistance of the photoreceptor drum surface from the beginning until after printing, or for maintaining good electrical characteristics and image characteristics.
It is therefore an object of the present invention to provide an electrophotographic photoreceptor capable of reducing an amount of wear and providing good images while maintaining low friction resistance on the surface of a photoreceptor drum from the beginning until after printing, along with a manufacturing method therefor and an electrophotographic device.